Kirill Yurovsky: Structure and principle of the rotary engine

Not all automotive connoisseurs know that at different times in different countries of the world, unusual rotary internal combustion engines have been put on cars. This unique unit has its own great history and, probably, good prospects for application in the future.

What is the Wankel rotary engine

It is a technically simple power unit. Instead of several pistons with rings and connecting rods, it has one triangular rotor mounted on a shaft. The shaft is not a crankshaft, but an eccentric one. The combustion chambers are arranged uniformly throughout the rotor’s rotation circle.

The rotary engine has more than twice as few parts as the piston version. There is no cylinder head with valve system in its usual form and the piston group itself. Significantly less weight and dimensions.

Currently there are 5 different types of rotary engines. They have significant design differences between each other. But the main principle is the same for all types – a rotor on an eccentric shaft instead of pistons on a crank mechanism  – says  Kirill Yurovsky.

History of the rotary engine

Power units with a rotor instead of a piston group received the stable name “Wankel engine”, after the surname of the inventor. In fact, several types of rotary motors, different from Wankel’s invention, have been developed around the world. But it was German engineer Friedrich Wankel who first started working in this area back in the 1920s.

The engine required components and parts, production of which is possible only with the use of high technology of metal processing, the most precise fitting, which at that time were certain difficulties. That is why it was not possible to put the product into mass production at once. By the time World War II started the necessity of tested serial products rather than experimental ones.

The work on the engine was completed already in France, where the equipment came from the defeated Germany, in 1957, at the company NSU under the direction of engineer Walter Freudet.

Application of the Wankel engine in the West and in the USSR

The first 57 hp rotary engine was installed in 1957 in the NSU Spyder sports car. It had an unbelievable speed for that time and such power of the engine – 150 km/hour.

In 1963, the rotary engines began to be used in mass-produced cars for the public. For several years they were put on Mercedes, Chevrolets and Citroens. But the engine showed a number of significant drawbacks. As a result, manufacturers returned to using classic, proven piston engines.

Japanese automakers were more persistent than others. They used rotary engines on some Mazda models. They eliminated weaknesses, increased engine life before overhaul, and reduced fuel consumption. However, for a variety of reasons, the Japanese have returned to the classic ICE. The last Mazda RX Spirit R with a rotary engine rolled off the assembly line in 2012.

In the USSR, the first domestically made rotary engine was put in 1974 on the legendary – VAZ 2101.

A special design bureau was organized to create it. The Wankel engine served as a prototype. About 50 prototypes were made and marked as VAZ 311. Those VAZ cars were not for sale to the general public, but were used by the State Automobile Inspection and KGB as official cars.

At first, the cars with this engine were admired for their power, acceleration dynamics, low noise and smooth running. But in a year only one car was left on the road. The engines of the rest broke down. The main reason for the breakdowns was the unreliability of the seals ensuring the sealing of the combustion chambers during fuel flashover.

Work on the domestic rotary engine continued, and powerful two-section VAZ 411 and 413 with 120 and 140 hp were created. “Zhiguli” with these engines again entered the service of the power structures.

This achievement of the Soviet car industry was not advertised. Rumors were circulating that KGB people were driving fast cars with incredible secret engines.

Then the VAZ 414 and 415 rotary engines were developed. These were more advanced universal units. They could be put both on the VAZ “eight” and “nine”, as well as on no less popular at the time “Moskvich” and “Volga”.

The last development of VAZ 415 was never used. Its predecessor, VAZ 414 since 1992 was put on the popular car model VAZ 2109 (“Sputnik”, “Samara”).

“Nines” with these engines had unusual characteristics. Acceleration to 100 km/h in 8 seconds, ability to work long hours at extremely high rpm. VAZ 414 consumed less fuel (14-15 liters per 100 km), than previous rotary engines (18-20 liters per 100 km). But still more than a piston engine.

However, at VAZ, too, the rotary ICE could not compete with the traditional ones, and soon their use was discontinued.

Cars with such engines are not produced today. It is possible that car manufacturers may carry out design work in this direction without publicizing it, secretly from competitors.

Design and principle of operation of a rotary engine

The principle of operation and construction of the rotary engine is simultaneously similar to that of a conventional piston engine and electric motor. Just like a piston engine, the rotary version has combustion chambers, fuel injection, exhaust and ignition systems. The design is similar to an electric motor in that the rotor receives power as it rotates inside the body. (Except for a rotary engine with a reciprocating shaft).

The electric motor receives kinetic energy by moving the electromagnetic field. Rotary ICE – by ignition of the fuel-air mixture and a sharp increase in pressure in the combustion chambers, as well as piston ICE.

There are 5 types of rotary engines known today:

  1. With reciprocating shaft motion. In these types of ICE, the rotor and shaft do not make a complete revolution around the axis
  2. Classic Wankel engine with planetary rotation of the shaft
  3. Engines in which the combustion chambers are arranged in a spiral
  4. Engines with uniform shaft rotation with combustion chambers arranged in a spiral without sealing elements
  5. Engines with pulsating rotation

Like reciprocating engines, rotary variants have four operating cycles:

  1. Fuel-air mixture injection
  2. Compression of the mixture
  3. Ignition
  4. Exhaust

In conventional piston engines, fuel injection and combustion chamber tightness are provided by the valve system and piston rings. In different types of rotary combustion engines, the sequence of strokes is ensured in different ways. In some combustion engines, the volume of the combustion chamber is reduced and the mixture is compressed due to the overlapping of the chamber by the rotor apex. Others have a mechanically actuated seal. But the principle of operation is the same for all types.

  1. The ignition of the fuel mixture increases the pressure in the combustion chamber many times.
  2. The pressure gives a kinetic impulse to the rotor plane and rotates it.
  3. The rotor transmits the torque through the shaft and toothed gear further to the mechanisms of the car. The rotor plane reaches the exhaust window, the window opens and exhaust gases are discharged into it.
  4. The cycle repeats.

Advantages and disadvantages

The rotary engine has a set of great advantages over the traditional piston engine.

The main advantage is the simplicity of design. Due to the absence of a piston and crank group of nodes, the rotary engine is almost twice as light and compact as a conventional engine. The light weight allows the load to be distributed evenly over the entire vehicle base. This improves drivability and increases dynamic performance of the vehicle.

  • Compactness allows for more interior space.
  • The rotor rotates smoothly, with no vibration from the explosion of the fuel mixture in each cylinder, producing power evenly.
  • With the same volume of combustion chambers, the rotary engine is much more powerful.
  • Simplicity of design and a minimum of parts make repairs easier.

Therefore, it seems that the entire world car industry long ago and completely had to give up piston engines in favor of rotary engines. But this did not happen. Consequently, the rotary version has a number of significant disadvantages, which today outweigh all of its pluses. The disadvantages are as follows:

  • The rotary engine consumes much more fuel. This is a major disadvantage in this day and age, when every automaker wants to make their car as economical as possible.
  • Increased oil consumption – 0.5 liters per 1,000 km of mileage. Oil replenishment is required every 4-5 thousand km. The lack of oil leads to an instant engine failure.
  • Production of the rotor and curved combustion chambers requires the highest technological precision on expensive ultra-precise equipment. This increases the cost of the engine.
  • A feature of lenticular combustion chambers is that they absorb more heat during operation. As a result, the engine is prone to overheating, boiling coolant in the cooling system, which interferes with the operation of the car and leads to an accelerated failure of engine parts.
  • The rotary engine has its own weak point. The seals, which ensure the tightness of the combustion chamber at the moment of ignition of the fuel mixture, cannot withstand the loads for a long time and fail. As a result the service life of the most perfect rotary engine without repair does not exceed 100 – 150 thousand km of run of a car.

In addition to economic and technical disadvantages, the rotary engine is simply unaccustomed to drivers and mechanics. The car with it drives differently. Due to the small mass of the engine, it has no reserve of inertial energy. At the slightest gas pedal reset the car quickly loses speed, which is good when braking, but uncomfortable when driving. You have to change gears more often. You can’t brake with this engine, a stalled engine even in first gear is easy to turn. Some people just don’t like the sound of the rotary engine running.

Perhaps this engine has a great future. The piston engine has come a long way in its evolution. Crankshafts and piston systems began as early as steam engines.

The rotary variant has not had such a long evolution and mass production, so it has shortcomings and weaknesses. It is important that the rotary engine can efficiently run on gas fuel, including hydrogen. This could open up great prospects for it in the future.